Thermal anemometers and pitot tubes have been used for many years for sensing fluid velocities within closed ducts. The pitot tube measures the difference between the dynamic pressure and the static pressure of a moving stream. This difference is proportional to the fluid velocity past the pitot tube. This pressure differential is proportional to fluid velocity. Thermal anemometers immerse a heater in a fluid stream. The heater transfers heat from a hot surface to the moving fluid stream. The amount of heat transferred by convection to the moving fluid stream is determined in part by the velocity of the moving fluid. Fluid flow velocity may be determined by providing a constant heat input and measuring the variation in temperature of the hot surface with respect to the fluid temperature (constant power method). Or alternatively, one may maintain the temperature of the hot surface at a constant, elevated value with respect to the fluid temperature and measure the heat input required to maintain the temperature elevation (constant temperature method).
Both pitot tubes and prior art thermal anemometers make a "local" measurement of fluid flow. Consequently, to compute mass flow rates within duct work, it is common to use multiple sensors to monitor flow at several locations along the cross-section of the duct.
There are three general forms of commercially available thermal anemometers for use in large ducts. In one form, a long thin sensing unit is constructed of flexible material and is formed to traverse the duct or pipe cross-section several times. The whole of the external surface of the anemometer exchanges heat with the fluid. Since more of the duct or pipe cross-section participates in heat transfer, a better estimate of average flow is obtained. These may be referred to as averaging anemometers. This type of sensor has a large thermal capacity which causes substantial errors when the fluid velocity changes. This type of prior art device is sometimes referred to as a "coaxial" device.
A second approach is to construct an array of multiple, small sensing units distributed throughout the duct or pipe cross-section. Signals from each of the sensing units are monitored and an average value is formed by a computing means. The principal disadvantage of this approach lies in it's complexity. Each sensing element has a heat source that must be controlled. Each sensing element must be individually monitored. Finally, a relatively complex computer must be used to first solve multiple equations for each of the sensing elements and then average those responses to obtain an estimate of the average flow velocity.
A third approach is to transport a small sensing unit to different locations throughout the duct or pipe cross-section. The flow measurement at each location is obtained and is stored or recorded. The average value is then computed from the stored measurements. This approach is not suitable for real-time measurement due to the mechanical complexity of the transport system and the time required to scan the duct.